Investigation into the regulatory mechanism of BRCA2 stability

Inherited mutations in the BRCA2 gene predispose individuals to the development of breast and ovarian cancers. The BRCA2 protein plays a fundamental role in the repair of DNA double strand breaks by homologous recombination (HR). BRCA2 mediates the recruitment of the RAD51 recombinase to DNA damage sites, which in turn promotes homologous pairing and strand exchange during HR. It has been reported that increased BRCA2 mRNA levels correlate with poor cancer prognosis, and recently it has been shown that increased levels of BRCA2 suppress HR. As HR is regulated through the cell cycle and can only be employed during S and G2 phases of the cell cycle, in this study, the cell cycle-dependent regulation of BRCA2, as a key player of HR, was investigated. In this study I report that BRCA2 stability is regulated by the ubiquitin-proteasome system (UPS), which has become increasingly evident as an important regulator of DNA repair. In line with this, I found that BRCA2 can be ubiquitylated in vivo and that it interacts with proteins of the UPS. Interestingly, I observed that BRCA2 levels and its ubiquitylation status change during the cell cycle. Using a siRNA-based approach, I identified a candidate E3 ubiquitin ligase, the SCFFBXW7 complex, which is also a known major cell cycle regulator. siRNA-mediated knockdown of FBXW7 led to stabilization of BRCA2 and overexpression of FBXW7 resulted in BRCA2 ubiquitylation in vivo. Furthermore, I have refined the regions that the SCFFBXW7 interacts with on BRCA2, which likely occurs in a phosphorylation-dependent manner. Taken together, these observations suggest that BRCA2 stability is regulated by the UPS in a cell cycle-dependent manner, which may be an important regulatory mechanism for BRCA2 function.